I. Field of the Invention
This invention is directed to the field of wireless communications. More specifically, this invention is directed to the handoff of wireless communication sessions between radio networks.
II. Description of Related Art
a. Wireless Communication Systems
In a typical wireless communication system, a mobile communication device (mobile station) will communicate with a fixed base station (e.g., access point) over an air interface using a radio network, and the base station provides connectivity with various resources, such as a transport network. When the mobile station first enters a coverage area of the base station (e.g., an area defined by a radio frequency (RF) radiation pattern of the base station) the mobile station will register (or associate) with the base station (access point) or other infrastructure of the particular radio network that includes the base station/access point. As long as the mobile station remains within the radio coverage area of the base station, the mobile station communicates with and through the base station so as to access the available resources.
Wireless communication systems take various forms. Two well known examples of such systems are cellular radio communication systems (e.g., code division multiple access (CDMA), time division multiple access (TDMA) or general packet radio service (GRPS) systems, which may be termed wireless wide area networks (WWANs)) and wireless local area networks (e.g., IEEE 802.11 (wireless Ethernet) or IEEE 802.15 (e.g., Bluetooth) networks.
In a WWAN, such as a CDMA network for instance, a carrier (service provider) will typically provide various radio access networks each serving a given geographic area. Each radio access network usually includes at least one base transceiver station (BTS) and at least one base station controller (BSC). The BTS radiates to define one or more coverage areas, such as cells or cell sectors, and the BSC then controls communications that occur in those coverage areas. In a usual arrangement, the BSC is then coupled with a mobile switching center (MSC) that provides connectivity with the public switched telephone network (PSTN) and/or a network access server (e.g., a packet data serving node (PDSN)) that provides connectivity with a packet-switched network such as the Internet.
When a mobile station powers on or otherwise enters a given coverage area of the cellular radio access network, the mobile station conventionally registers with the radio access network through signaling with the MSC and/or other network infrastructure entities. The mobile station may then engage in communication through the radio access network, so as to access resources such as the PSTN and the Internet.
A wireless local area network (WLAN) such as an 802.11b system, on the other hand, includes one or more access points that sit as nodes on a local area network (LAN). Each access point radiates to define a given radio coverage area of the access point. When a mobile station powers on or enters the coverage area of a given access point, the mobile station “associates” (e.g., registers) with the access point and gains connectivity with the LAN via the access point. The mobile station may then access resources that are available on or through the LAN, such as Internet connectivity or Voice over Internet Protocol (VoIP) services, for example.
WWANs typically serve public areas. That is, for example, a mobile station served by a typical cellular radio communication system is usually able to access the WWAN system from anywhere within the geographic coverage of the system, whether the mobile station is on the road, in a park, in a building, or elsewhere. To provide this extent of coverage, the cellular carrier typically positions BTSs in centralized locations, such as alongside roads or in the middle of cities or other areas.
WLANs, on the other hand, typically serve private areas, such as the inside of a given building, for instance. To provide this sort of coverage, an individual or WLAN provider typically positions one or more wireless (radio) access points within the building, with antennas directed as much as possible within the building. In almost all WLAN setups, some coverage leaks outside of the building as well. It will be appreciated that the configurations described above are merely typical and other arrangements are possible.
b. Handoff
When a mobile station moves between wireless coverage areas, the mobile station will normally be handed off from one coverage area to another. Such a handoff may occur intrasystem, such as between WWAN (e.g., CDMA) sectors/cells or between WLAN coverage areas. Alternatively, a handoff may occur intersystem, such as between a CDMA sector and a WLAN coverage area (provided the mobile station is capable of operating in both systems), such as is described in currently pending U.S. patent application Ser. No. 10/779,261 to Jones et al., filed Feb. 14, 2004, which is also assigned to the assignee of the present application, Sprint. The entire disclosure of U.S. Ser. No. 10/779,261 is herein incorporated by reference.
In order for such an intersystem handoff to occur, however, some mechanism must be in place to trigger the handoff, i.e., to indicate when a handoff should occur. Various triggering mechanisms may be used for this purpose. By way of example, such triggering mechanisms include (i) received signal strength, (ii) round trip signal delay and (iii) geographic location, any of which could be carried out by the mobile station itself, by the wireless network infrastructure, or by a combination of the mobile station and the infrastructure.
Received signal strength refers to the strength of signals (or the energy-to-noise ratio of signals) that the mobile station receives from the radio network infrastructure in various coverage areas. Higher received signal strength in a given coverage area often indicates that the mobile station would be better served by that coverage area. Thus, it is appropriate to use received signal strength as an indication of when to hand off from one coverage area to another.
To use received signal strength as a handoff trigger, such as when the mobile station is operating in a given coverage area and begins receiving signals (e.g. pilot signals or the like) from a neighboring/overlapping coverage area, the mobile station and/or infrastructure may monitor the strengths of the signals in order to determine when to effect a handoff. When a determination is made that the signal strength in a neighboring coverage area is sufficiently higher than the signal strength in the currently serving coverage area (e.g., the radio network the mobile station is currently being served by), the mobile station may then be handed off from the currently serving coverage area to the neighboring coverage area.
Round trip signal delay refers to the amount of time that it takes for a round trip communication between the mobile station and the infrastructure of a given coverage area. A shorter round-trip-delay for communication with the infrastructure of a given coverage area often indicates that the mobile station would be better served in that coverage area. Therefore, it is appropriate to use round-trip-delay as an indication of when to hand off from one coverage area to another, such as is described in U.S. Pat. No. 6,246,673 to Tiedemann Jr. et al., issued on Jun. 12, 2001. The entire disclosure of U.S. Pat. No. 6,246,673 is herein incorporated by reference.
To use round-trip-delay as a handoff trigger, such as when the mobile station is operating in a given coverage area and begins moving into a neighboring coverage area, the mobile station and/or the wireless infrastructure may monitor a round-trip-delay time in order to determine when to effect a handoff. For instance, the mobile station could send a time-stamped query signal to a base station and measure how long it takes to receive a response from the base station. Or the base station could send a time-stamped query signal to the mobile station and measure how long it takes to receive a response from the mobile station. When a determination is made that the round-trip-delay associated with a neighboring coverage area is sufficiently less than the round-trip-delay associated with a currently serving coverage area, the mobile station may then hand off from the currently serving coverage area to the neighboring coverage area.
Geographic location refers to the physical location of the mobile station within the geographic scope of various coverage areas. By maintaining data in the radio network and/or mobile station that demarcates borders between coverage areas, geographic location may be used as a trigger to effect handoff between the coverage areas. In practice, the mobile station or wireless network infrastructure monitors the geographic location of the mobile station (such as through use of a global positioning system (GPS) receiver included in the mobile station) and compares the mobile station's current location with the known geographic scope of various coverage areas. When a determination that the mobile station has entered the geographic region of a new coverage area is made, the mobile station may then hand off to that new coverage area. Such techniques are described in U.S. Pat. No. 6,321,090 to Soliman, which is herein incorporated by reference in its entirety.
Because of the accuracy of GPS location techniques in determining the location of a mobile station, such techniques are becoming increasingly more common for use in cellular communication systems for making call management decisions, such as when to effect a handoff from one radio network to another (e.g., intrasystem or intersystem). However, the use of GPS techniques for determining when to effect a handoff for in-building systems is problematic. For example, the physical structure of the building may prevent reception of signals from GPS satellites, thus making it difficult or impossible to accurately determine the location of a mobile station when positioned in a building. In such a situation, handoffs of the mobile station (e.g., communication sessions associated with the mobile station) may not be accomplished in a timely fashion (or accomplished at all), thus resulting in a potential loss of connectivity of the mobile station with a serving radio network (e.g., a dropped call).